Personal protection from exposure to harmful chemical and biological agents is often of concern to firefighters, medical practitioners and similarly situated personnel. Such protection often includes the use of apparel that provides a barrier to such agents. Butyl rubber is often used in standard protective clothing. However, garments made from butyl rubber are bulky and nearly impermeable to air and moisture (I. Lee, Yang and Wilusz; Polymer Engineering & Science, 1996, 36, 1217) resulting in unbearable levels of heat inside the garments during use. Other protective apparel includes textiles coated with polymeric materials to provide a chemical and/or biological barrier.
Various permeable materials having a wide range of mechanical and transport properties are known. Depending upon the particular application in which the permeable material is to be employed, however, certain combinations of properties are required. For example, in a protective apparel application, it is desirable that the material may transport water vapor and block the transport of harmful chemicals and/or biological agents, and be lightweight and flexible over a broad temperature range. A need exists for a material that can be a flexible, solid material with membrane characteristics that facilitate the transport of water vapor, for example, from a wearer of membrane-containing apparel to the atmosphere; allow moisture to permeate the garments to the extent necessary to afford comfort to the wearer, thus reducing heat stress; and block entry of certain chemical compounds and biological agents.
Equipment is often wrapped or packaged with film or fabric tarpaulins, hoods or other covers to prevent surface damage during transportation and storage. These covers may be prepared from highly moisture impermeable films and fabric.
Many relatively small items are shipped on pallets, that is, platforms that are easily moved by forklifts or small cranes. Pallets provide convenience in loading and unloading goods from shipping containers, and in moving smaller amounts of goods over shorter distances, such as in warehouses or to deliver a retail quantity. The small items may be unpackaged or packaged, for example in bags or boxes, when they are placed on the pallets.
A loaded pallet preferably has integrity and stability so that the goods are not damaged or lost during shipping. To provide the necessary integrity and stability, the pallet and its load have been typically wrapped together in film, for example overlapping layers of polyethylene stretch wrap that may be applied by machine or by hand; see, e.g., U.S. RE38429. Other generally practiced methods of providing integrity and stability to loaded pallets include wrapping the pallet and its load in heat shrinkable film, encasing the loaded pallet in a sheath or “hood” which may be heat shrinkable or stretchable, and containing the goods in a single carton or box. These methods are sometimes referred to, individually or collectively, as “pallet unitizing”.
Using barrier films for wrapping small objects or articles in sealed bags is generally suitable since the object may be dried before being sealed in the bags and/or drying agents may be included inside the sealed bags. This approach is less suitable for large objects such as vehicles, boats, motors, machinery, industrial goods, pallets or containers holding smaller articles, and other bulky equipment because the covers are typically not hermetically sealed around the object and thorough drying of the object may not be feasible. This may be especially problematic during storage or when shipping by ship or railroad, because the large objects may be exposed to adverse weather conditions for long periods of time. Atmospheric moisture and/or rain may enter the space under the cover and be trapped and condense. With high barrier covers, there is no way for water to permeate back outside the cover, resulting in a buildup of moisture inside the cover, leading to the possibility of corrosion.
Large amounts of money are lost each year because of corrosion of, for example, iron, steel, and other metals. There are many factors affecting corrosion rate including moisture, oxygen, and salt presence. A common corrosion occurs due to electrochemical reactions at high humidity conditions. For example, when iron is exposed to moist air, it reacts with oxygen to form rust (iron oxide). The result of corrosion may be the formation of metal oxide that flakes off easily, causing extensive pitting thereby causing structural weakness and disintegration of the metal. Corrosion can also affect other properties of metal parts such as reducing conductivity or increasing surface roughness so that moving parts become unable to move freely.
In addition to corrosion of metals, mold growth may occur in the condensed moisture on the surface of the equipment.
Using a film or cover with a high water vapor transmission rate can prevent condensation of water inside the cover by allowing equilibration of the trapped moisture back into the surrounding atmosphere. Using such a film prevents or reduces rust formation and corrosion and reduces the opportunity for mold growth.
Many previous permeable membranes are microporous (i.e., permeable due to the presence of microscopic pores through which vapor can pass). Microporous membranes, which may be laminated on or between nonwoven textiles, have increased permeability, but may not provide adequate barriers to liquids because of their nonselective permeability. Liquids under pressure may be able to penetrate the pores. Most microporous films are biaxially oriented, so only a small amount of shrinkage is possible, and they cannot be shrunk without losing their porosity. They may also have low tear strength and their surfaces may be easily fouled, thereby losing permeability.
Various references describe semipermeable materials produced from a variety of polymers that may be useful for protective covers (e.g., U.S. Pat. No. 6,579,948). Thermoplastics with high moisture permeation are used for a wide range of applications, such as surgical garments, films for textile lamination for sports and leisure. Hydrophilic block copolymers, such as poly(ether-co-amide) (Pebax®) and poly(ether-co-ester) (Hytrel®) are preferred materials, where the soft polyether blocks provide hydrophilic and flexibility and the hard polyamide blocks or polyester blocks enhance mechanical integrity. Recently protective fabrics comprising a selectively permeable membrane comprising organic acid-modified ionomer compositions have been disclosed (U.S. Pat. No. 7,514,380).
However, those polymers are not free of problem in applications. One of the major disadvantages of block polyethers such as Pebax® and Hytrel® is a lack of compatibility to polyolefin. The block copolymers have poor adhesion to polyolefins and are difficult to blend with polyolefins. For example, Pebax® is difficult to adhere to polyethylene- and polypropylene-based nonwoven fabrics. Also the melting temperature for Pebax® can be too high for laminating with polyolefin based substrates without causing deformation. These factors significantly restrict the use of the block copolyethers in protective fabric applications.
Because no single material has emerged which satisfies all of the technical requirements and that presents a cost-effective alternative, it is desirable to provide a selectively permeable membrane or structure or layer that displays a combination of mechanical properties, low temperature flexibility, selective transport, ease of processability, and cost-effectiveness, so as to render it suitable for use as a protective cover for objects that limits corrosion and/or mold growth. There is also a practical need to have a breathable material that is compatible with polyolefins.